Merge pull request #2 from lomereiter/master

Added Mac OS X build for sambamba
samtools, bcftools, htslib: builds for latest release candidates. Conflict with current science/tabix and science/samtools due to changed location of bgzip and bcftools. Work towards providing support for new bcftools in bcbio-nextgen (chapmanb/bcbio-nextgen#155)
Ensure htslib dependency available for bcftools. Fixes chapmanb/bcbio-nextgen#213
Explicitly define bcftools and samtools versions to avoid issues with non-GNU make builds. chapmanb/bcbio-nextgen#219
Swap samtools formula to 0.19 release without bcftools to avoid overlap with bcftools 0.20 releases. chapmanb/bcbio-nextgen#221
bump sambamba version